1. Field of the Invention
This invention relates to a method for producing conjugated linoleic acid. More particularly, this invention relates to a method for efficiently transforming the linoleic acid contained in a fat or oil into conjugated linoleic acid by carrying out the alkali isomerization of the linoleic acid in a specific solvent.
2. Background Art
As a method for producing conjugated fatty acids, the so-called "alkali isomerization" method is known, in which an organic solvent, typically ethylene glycol, is employed (J. Am. Oil Chem. Soc., 36, 631 (1959); The 34th Annual Meeting on Oil Chemists' Society, p. 171 (1995); and "Standard Methods for the Analysis of Fats, Oils and Related Materials", 2.4.16-17). It is reported in the J. Am. Oil Chem. Soc., 36, 631 (1959) that, when methyl linolenate was heated in a potassium hydroxide-ethylene glycol solution at 200.degree. C. for 7 hours, about 80% conversion (conjugation) was attained. However, the reported method also involves cyclization and other side reactions.
According to the 34th Annual Meeting on Oil Chemist's Society, p. 171 (1995), tests were carried out in accordance with the method described in the "Standard Methods for the Analysis of Fats, Oils and Related Materials", 2.4.16-17, and the following are reported: when methyl linolate was allowed to react in a potassium hydroxide-ethylene glycol solution at 180.degree. C. for 2 hours, the rate of the conjugated diene formed reached approximately 80% or more, and the amount of potassium hydroxide used in this reaction was six times the number of moles of 1,4-butadiene structure; and, when a safflower oil was subjected to isomerization reaction at 30.degree. C. for 1.5 hours by using, as solvents, dimethyl sulfoxide and dimethylformamide, and, as an alkali, sodium methoxide (in an amount of two times the number of moles of 1,4-butadiene structure), the rate of the conjugated diene formed reached approximately 73%.
Among the three types of solvents reported in the above reference, ethylene glycol is most preferred from the viewpoint of alkali solubility (in the case of the other two solvents, the type of alkalis which can be used is limited, and, the solvents must be used in a large amount).
As discussed above, in the conventional alkali isomerization methods for producing conjugated fatty acids, ethylene glycol, dimethyl sulfoxide or dimethylformamide is used as a solvent. However, these three compounds all have some toxicity. The conventional methods thus have the drawback that the resulting conjugated products cannot be used for foods.
It is therefore an object of the present invention to provide a method for producing conjugated linoleic acid, which can more efficiently transform linoleic acid into conjugated linoleic acid and which makes it possible to use the conjugated product in the field of foods.